Antagonism (phytopathology)

Antagonism is often employed as a natural method to protect plants from diseases. This can occur through mechanism such as competition for space and nutrients, the production of toxins or siderophores by one pathogen to suppress another, induction of host resistance, or other processes that inhibit the growth or reproduction of pathogens, as demonstrated in the Ascochyta blight complex on peas.

Mechanism

Antibiosis

The mechanism of antibiosis involves an interaction between two organisms, where one organism produces substances such as toxins, enzymes, or antibiotics that harm another organism, particularly pathogens. These interactions reduce pathogen viability of pathogens, limit the spread of disease, and enhance plant protection. For instance, antagonistic microbes can produce lytic enzymes, such as chitinases, which break down the cell walls of fungal pathogens, effectively inhibiting their growth. Another example is Pseudomonas aeruginosa, which shows antagonism against Cladosporium. Such organisms are of great practical importance as they often produce antibiotics that modify normal growth processes.

Hyperparasitism

The mechasism of hyperparasitism involves a parasitic relationship where one organism, typically a fungus, directly attacks another pathogenic fungus. The hyperparasite physically interacts with the host, penetrates its cell wall, and extract nutrients. An example is the hyperparasitic fungus, Cladosporium cladosporioides, which parasitizes Puccinia striiformis f. sp. tritici (Pst), the causal agent of wheat stripe rust.

Induction of resistance

Induction of resistance in antagonism refers to the ability of an antagonistic bacterium to activate a host's defense mechanisms through biochemical changes, such as the production of pathogenesis-related proteins or antimicrobial compounds, thereby enhancing the plant's immunity locally or systemically against pathogens. For example, Bacillus subtilis induces systemic resistance in plants by triggering the production of pathogenesis-related proteins and antimicrobial compounds, offering protection against fungal pathogens such as Botrytis cinerea.

Competition for nutrients and space

Competition for resources occurs as antagonistic bacteria colonize the host and outcompete phytopathogens for essential nutrients, such as carbon sources like sucrose, fructose, and glucose, thereby reducing spore germination and the pathogen's ability to invade the host. For example, the yeast antagonist Rhodotorula glutinis deprives pathogens of critical micronutrients, such as iron, by producing siderophores.